Gas-liquid mixing systems are conventionally used for many purposes, such as the mass transfer of gases into and/or out of liquids. Oxidation and reduction reactions often require that a gas, such as oxygen, chlorine or hydrogen, be mixed with liquids in the presence of solids. Unwanted gases dissolved in liquids can be stripped from the liquid by mixing a desired gas into the liquid. Direct contact heating of a liquid requires that a hot gas be mixed into a liquid, and, in some instances, the pH of a liquid can be adjusted by mixing a gaseous acid or base into the liquid.
For example, oxygen gas is often mixed with various liquids. Oxygen gas can be mixed with activated sludge to aerate waste material and assist in digestion, it can be used to oxidize carbon, sulfur and/or nitrogen containing material in a liquid, it can also be mixed with liquids containing organic compounds to oxidize the organic compounds into alcohols, aldehydes and acids, or it can be mixed with hydrometallurgical process liquids to achieve various desired effects. Oxygen gas can also be mixed with liquids to reduce nitrogen-containing compounds into nitroso-containing materials, nitrites and/or nitrates. Oxygen gas can be mixed with liquids to reduce sulfur-containing compounds into disulfides, sulfoxides and/or sulfates.
The formation of hydrogen sulfide can occur in any aquatic based system containing sulfates in which the dissolved oxygen does not meet the oxygen demand. Even small quantities of hydrogen sulfide can produce objectionable odors thereby necessitating that oxygen be mixed into the liquid. Industrial and municipal wastewater can also be treated by biological treatment techniques in which aerobic microorganisms convert contaminants into carbon dioxide gas and biomass. Sufficient oxygen must be provided to the aerobic organisms in order to carry out the necessary biological processes, chemical oxidation and/or fermentation processes.
Hydrogen gas can also be mixed with various liquids or liquid solid mixtures. For example, hydrogen gas can be used to saturate carbon-carbon double bonds and to reduce nitro and nitroso compounds in organic materials. Hydrogen gas can also be mixed into liquids present in vegetable oils processing, yeast production, vitamin C production, coal liquefaction, and the production of other types of unsaturated organic liquids. Chlorine gas can also be mixed with organic and inorganic liquids. Carbon monoxide gas can also be mixed with liquids containing organic compounds. In each of these examples, gas can be mixed into a liquid to dissolve and react with the liquid and/or liquid solid mixture to achieve various desired effects.
Conventional gas-liquid mixing systems can be typically classified as either surface aerators or diffused gas delivery systems. Diffused gas delivery systems that require gas compression typically comprise coarse, medium or fine bubble diffusers, liquid motive force venturi, jet type mixers that require large pumping systems, or agitators that utilize hollow members or spargers positioned to deliver pressurized gas to a mixing zone. Diffused gas delivery systems that do not require gas compression equipment typically comprise self-inducing systems such as venturi systems, vortex systems, and rotor/stator pitched blade turbine reactors.
In traditional systems, the delivery of gas to the desired liquid depth requires the use of fans, blowers, compressors, venturi or vortex systems to entrain the gas or compress the gas to a pressure equal to or greater than the static head at the desired liquid depth. Some traditional systems deliver compressed gas to a porous material, such as a fine hole matrix, mesh or membrane, that is permanently mounted near the bottom of a tank to disperse gas. However, these porous materials are easily fouled and can become blocked when placed in dirty liquids, liquids having a high particulate concentration or high soluble mineral concentration. Fouled materials reduce efficiency, increase operational energy cost, and increase bubble size. Porous materials can also stretch over time, thereby increasing hole size and bubble formation diameter, or harden, thereby causing increased pressure. Larger bubbles, caused by larger hole size, increased pressure or fouling, reduce the available gas-liquid surface area, which reduces the overall Standard Aeration Efficiency (SAE). The efficiency of fouled, blocked or stretched materials can drop to only 30% to 40% of their stated SAE in clean water.
To remedy the higher energy costs associated with fine bubble diffusers, additional energy, maintenance and/or replacement equipment is often needed. Periodic cleaning and maintenance often involve expensive and hazardous HCl injections into the diffuser system and/or the emptying of the aeration vessel followed by physical cleaning. Plastic membranes must be periodically changed, which increases labor, materials and processing costs associated with an aeration system shut-down during installation.
Non-mechanical diffused gas-liquid mass transfer systems, especially those using fine bubble diffusers, can deliver standard aeration efficiency (SAE) of 1.6 to 7 kilograms of dissolved oxygen (DO) from air per kilowatt-hour (kg/kWh) in clean water (SAE-ANSI/ASCE Standard 2-91). Their efficiency, even when clean, is frequently reduced by the intensity of the liquid mixing. The efficiency of a non-mechanical diffused gas-liquid mass transfer system in dirty or contaminated liquid can be only 40 to 50% of the clean water efficiency of the system.
Some examples of diffused aeration systems that are not based on fine bubble diffusers include traditional mechanical diffused aeration systems. Traditional diffused aerator systems can include a high speed prop mixer and a regenerative blower, such as the commercially available Aire-O2 Triton®, large liquid mixers systems using a gas compressor, such as the draft tube aeration system commercially available from Philadelphia Mixers Corp., and jet aeration systems using a gas/liquid mixing jet, a liquid pump and a gas compression device, such as the system commercially available from US Filter Corporation.
Other traditional mechanical diffused aeration systems do not use a compressor, however, these systems require a vortex or a venturi system to create gas pockets at some depth below the surface of the liquid. Examples of these traditional mechanical diffused aeration systems include: U.S. Pat. No. 6,273,402 for a Submersible In-Situ Oxygenator, U.S. Pat. No. 6,145,815 for a System for Enhanced Gas Dissolution Having a Hood Positioned Over the Impeller with Segregating Rings, U.S. Pat. No. 6,135,430 for Enhanced Gas Dissolution, U.S. Pat. No. 5,916,491 for Gas-Liquid Vortex Mixer and Method, and U.S. Pat. No. 5,925,290 for Gas-Liquid Venturi Mixer, each of which are incorporated by reference herein.
In each of these traditional gas-liquid mixing systems that do not require a compressor, either liquid pumps or mixers are required to create high liquid velocities within the system. In order to introduce gas into the system, a velocity head must be created that is greater than the static head at the desired liquid depth at which the gas is introduced to the liquid. To overcome this static head, traditional systems require a liquid moving device, such as an axial or radial liquid pump or mixer, to accelerate a volume of liquid at a high velocity within a tank or holding area.
Conventional mechanical diffused air systems typically have an SAE of from 0.4 to 1.6 kg/kWh. Typically, low speed surface aerators give the highest SAE for mechanical aeration systems. These systems typically state an SAE of from 1.9 to 2.5 kg/kWh. However, surface aerators achieve low gas utilization and require large volumes of gas to be mixed with liquid, causing a high rate of off-gassing, which strips volatile organics from the liquid into the gas.
The present invention has been developed in view of the foregoing and to remedy other deficiencies of related devices.